1. Field of the Invention
The present invention relates to: an antenna with which a digital signal or an analog high-frequency signal, e.g., that of a microwave range or an extremely high frequency range, is transmitted or received; and a driving method thereof.
2. Description of the Related Art
Various techniques have been proposed over the years for changing the directivity of an antenna and subjecting an emitted beam for scanning. For example, some methods, e.g., adaptive arrays, allow a signal which is received via a plurality of antennas to be processed in a digital signal section to equivalently realize a beam scanning. Other methods, e.g., sector antennas, place a plurality of antennas in different orientations in advance, and switch the main beam direction through switching of a path on the feed line side. There are also methods which place reflectors and directors (which are unfed elements) near an antenna to tilt the main beam direction.
The slot antenna is one of the most basic resonant antennas, and is a promising antenna in terms of applications to wideband communications because it is expected to provide bandwidth ratio characteristics of about 10% in the case where the slot length corresponds to a ½ effective wavelength and at least 15% or more in the case where the slot length corresponds to a ¼ effective wavelength. These values are wideband as compared to a bandwidth ratio of about 5% of a patch antenna, which is a similarly basic resonant antenna.
Japanese Laid-Open Patent Publication No. 2003-527018 (hereinafter “Patent Document 1”) discloses, as a sector antenna utilizing a slot antenna, a sector antenna structure in which a plurality of slot antennas are radially placed to realize switching of the main beam direction through switching of a path on the feed line side. In Patent Document 1, a Vivaldi antenna which is known to have ultrawideband antenna characteristics is used as an antenna to realize global switching of the main beam direction of emitted electromagnetic waves having ultrawideband frequency components.
Moreover, Japanese Laid-Open Patent Publication No. 2005-210520 (hereinafter “Patent Document 2”) discloses an example of a variable antenna which employs unfed parasitic elements for tilting a main beam direction in which emission from a radiation slot element occurs. In the variable antenna shown in FIG. 20, in proximity, a ½ effective wavelength slot resonator which is excited by a feed line 115 as a radiator 201 and unfed slot resonators serving as parasitic elements 203a and 203b are placed on a ground conductor 101. Through adjustment of the slot lengths of the parasitic elements 203a and 203b, switching can be made as to whether the parasitic elements function as directors or reflectors relative to a reflector, thus varying the direction of an emitted beam from the radiator. In order to allow the parasitic elements 203a and 203b to function as directors, the slot lengths of the parasitic elements may be adjusted to be shorter than the slot length of the radiator. In order to allow the parasitic elements 203a and 203b to function as reflectors, the slot lengths of the parasitic elements may be adjusted to be longer than the slot length of the radiator. In order to adjust a slot length, a slot length which is longer than necessary is prescribed on the circuit board; and, in a state of allowing the element to function as a slot circuit with a short slot length, somewhere along the slot length, selectively conduction is achieved by means of a switching element 205a or 205b so as to astride the slot along the width direction between portions of ground conductor. Patent Document 2 mentions use of MEMS switches as an exemplary method of implementing the switching elements 205a and 205b. 
An antenna for a mobile terminal device for fast communications not only needs to be downsized, but also needs to be able to vary the main beam direction of electromagnetic waves emitted therefrom, in order to avoid interference waves such as reflected waves. However, conventional slot antennas have the following problems.
Firstly, in the antenna disclosed in Patent Document 1, four slot antennas, most of whose constituent elements are not shared, are radially placed within the structure, and a driving method is used which switches the feed circuit for each slot antenna, whereby a function of switching the main beam direction is realized. However, there is a problem in that the antenna structure is large.
Secondly, in the antenna disclosed in Patent Document 2, too, slot antennas whose constituent elements are not shared are placed in parallel, thus presenting a problem from the standpoint of downsizing. Moreover, there is only a limited frequency band in which the slot antennas to be used as parasitic elements function as directors or reflectors, thus resulting in a problem in that the main beam direction of the antenna may possibly change to a different direction within the operating frequency band. Thus, the antenna disclosed in Patent Document 2 may be applicable to a narrow-band communication system, but is difficult to be applied to a communication system where a wide frequency band is required for performing high-speed transmission. To be more specific, firstly, the ½ effective wavelength slot resonator has a radiation band of about 10%, which makes it necessary to adjust the slot length of each parasitic element so as to operate at a frequency which is different by 5% or more from the center frequency of the operating band. Secondly, it is necessary to maintain a degree of coupling between the radiator and the parasitic elements at an upper limit frequency and a lower limit frequency of the operating band. However, coupling between the slot resonators tends to lower as the difference between their resonant frequencies increases, and therefore it is difficult to simultaneously satisfy the above two conditions. Moreover, the antenna disclosed in Patent Document 2 may be capable of tilting the main beam direction, but is not able to realize drastic switchability, e.g., invert the main beam direction.